Gas streams comprising hydrogen sulphide are typically produced as waste products or by-products from many industrial processes. For example, acid gas streams comprising mainly hydrogen sulphide are typically produced during oil refinery operations in which sulphur is removed from crude oil. It is necessary to treat such hydrogen sulphide-containing streams before discharging them to the atmosphere so as to reduce or remove altogether their content of sulphur-containing gases. One well known, widely practised process for treating a gas stream comprising hydrogen sulphide is the Claus process. The Claus process is well known and is discussed, for example, in EP 0,237,217.
Sulphur recovery units based on the Claus process produce elemental sulphur from feed gases with a high concentration of H2S by partial oxidation of the latter using air, oxygen enriched air or pure oxygen as the primary oxidant. The sulphur dioxide produced by this oxidation exothermically reacts with a portion of the remaining H2S to produce sulphur vapour and water vapour. Cooling of the process gas downstream of the thermal Claus step as well as downstream of each catalytic Claus reactor leads to condensation of the majority of the sulphur vapour carried by the process gas. This allows for separation of the liquid sulphur product from the process gas. The different streams of liquid sulphur are collected in a container/sulphur pit. Sulphur obtained in this way thus contains appreciable amounts (up to 500 wt.-ppm of physically as well as chemically dissolved H2S which poses a considerable toxic/explosion/fire risk in storage and transportation operations in which H2S inevitably gases out into the headspace of the container/pit. In order to reduce this risk, a large proportion of the dissolved H2S is removed from the liquid sulphur as early as possible in the supply chain. Usually, the removal is effected from the sulphur pit(s).
Respective technologies applied according to the state of the art use gases such as air or nitrogen to either sweep the headspace of the containment pit or degas the liquid sulphur by passing the gas through the liquid sulphur. These methods produce an off-gas stream, for example a gaseous stream comprising nitrogen or air and containing components such as H2S, SO2, COS, CS2 and sulphur vapour.
These off-gases (also known as sweep gas or pit gas) are most often sent to the incinerator/stack section of the Claus unit, where all sulphur components are thermally or catalytically oxidised to SO2, thereby contributing to the overall SO2 emission of the sulphur recovery plant. In cases where very high sulphur recovery efficiency of >99.8% is required, the above described SO2 contribution originating from the sulphur pit must be reduced or removed and therefore other methods must be applied.
Such methods include treating the off-gas streams in a caustic scrubber system thereby eliminating most of the sulphur components by incorporating them into the liquid phase in the form of, for example, sulphides, sulphites, thiosulphates, sulphate and elemental solid sulphur. Alternatively, more recently, rather than treating the off-gas in a caustic scrubber system, the off-gas may be recycled up-stream of the thermal stage of the Claus unit, thereby avoiding the need to use chemicals such as aqueous NaOH and/or salts which generate S2− or HS− ions in the aqueous liquid phase. However, in order to recycle the off-gas (which has not been degassed at an elevated pressure), it must be pressurised, for example by means of a steam ejector, to a considerable degree: i.e. from almost atmospheric pressure up to greater than the pressure of the process gas (which is typically around 1.5 bar (0.15 MPa). In addition, if sufficient precautions are not taken, sulphur vapour from the off-gas will condense and even solidify in the burner internal components within the thermal reaction region which can be highly detrimental and jeopardise reliable long-term operation. This recycling of the off-gas to the section upstream of the thermal Claus stage ensures that the potentially O2-containing off-gas is depleted of all molecular oxygen by the time it exits the first thermal-reaction-region. This is important in the prior art methods because molecular oxygen must not be allowed to contact the commonly used Al2O3-based Claus catalyst.
There, therefore, a desire to provide an improved gas treatment process and/or apparatus which mitigates at least some of the problems associated with the prior art.
It is an aim of the present invention to provide an improved process and apparatus for recovering sulphur from a gas stream comprising hydrogen sulphide.
In particular, the present inventors have found that the process can be improved by recycling the off-gas downstream of the first thermal-reaction-region and upstream of one of the catalytic converters into the process gas pipe.